1. Field of the Invention:
This invention relates to a variable emphasis circuit suitable for a noise reduction apparatus for improving the signal to noise ratio.
2. Description of the Prior Art:
A noise reduction apparatus is usually used in various signal processing circuits for the purpose of improving the signal-to-noise ratio, and it is particularly important for sound multiplexing in the FM broadcasting or television broadcasting.
FIG. 5 shows the frequency spectrum of the TV sound multiplex system in U.S.A. As the baseband signal a sum L+R of signals, which is a first sound signal of stereo, is transmitted. The difference combination L-R of signals, which is a second sound signal of stereo, is transmitted as an amplitude modulation of a carrier 2f.sub.H (f.sub.H being the horizontal scanning frequency).
As a third sound signal, a frequency modulation of a carrier at 5f.sub.H is transmitted.
In the case of such sound multiplexing, the signal-to-noise ratio of the difference combination L-R of signals and the frequency modulation as the third sound signal is deteriorated particularly in a weak electric field range. Therefore, it is necessary to provide a noise reduction apparatus for these signals.
FIG. 6 is a block diagram showing such a noise reduction apparatus. Reference numeral 10 designates a compression system for compressing the dynamic range of the sound signals to be transmitted, and numeral 20 an expansion system for expanding the received signal to recover the original sound signals. A sound signal supplied to an input terminal 1 is first fed to a fixed emphasis circuit 2, which provides a preemphasis to a high frequency range where the noise component is high. The output of the fixed emphasis circuit 2 is fed to a voltage controlled variable gain amplifier 3 (hereinafter referred as VCA). Reference numerals 4 and 5 designate an effective value detector and a band-pass filter, respectively, for controlling the gain of the VCA 3. When the dynamic range of signal is large, the VCA 3 is controlled for compression. Reference numeral 6 designates a variable emphasis circuit for controlling only high frequency range gain to be described later, numerals 7 and 8 an effective value detector and a band-pass filter in the high frequency range, respectively, for generating a signal for controlling the variable emphasis circuit 6, and numeral 9 an output terminal.
The expansion system 20 on the receiving side likewise includes an input terminal 11, a variable emphasis circuit 12, a band-pass filter 13, an effective value detector 14, a VCA 15, a band-pass filter 16, an effective value detector 17, a fixed emphasis circuit 18 and an output terminal 19.
In such a noise reduction apparatus, the signal with preemphasis provided in the compression system 10 is subjected to a deemphasis process in the receiving side expansion system 20 for the improvement of the signal-to-noise ratio as is well known in the art. To this end, the band-pass filters 5 and 8 and effective value detectors 4 and 7 in the compression system and the band-pass filters 13 and 16 and effective value detectors 14 and 17 in the expansion system must have identical characteristics. Also, the VCA 3 and variable emphasis circuit 6 in the compression system must have opposite transfer characteristics to those of the VCA 15 and variable emphasis circuit 12 in the expansion system.
In this noise reduction apparatus, the variable emphasis circuit 6 in the compression system provides a greater emphasis to the high frequency range when the signal level is lower while it attenuates the high frequency range when the signal level is higher. Thus, when the input signal level is low the noise in the high frequency range can be suppressed, while when the input signal level is high, saturation in the high frequency range can be avoided. The noise suppression effect thus can be increased compared to a noise reduction system of the sole gain control type.
FIG. 7 shows the principles of the prior art variable emphasis circuit 6 (or 12). Reference numeral 21 designates an input terminal, numeral 22 a first weighting circuit to provide an emphasis to a high frequency component, and numerals 23 and 26 adders.
Reference numeral 24 designates a VCA, the gain A of which is controlled according to a signal from a control terminal 25. Reference numeral 27 designates a second weighting circuit having an opposite transfer characteristic to that of the first weighting circuit 22.
Reference numeral 28 designates an output terminal.
In this circuit, the transfer function H(s) between the input and output with respect to the gain A of the VCA 24 is given as ##EQU1## where T(s) is the transfer characteristic of the first weighting circuit 22 and 1/T(s) is the transfer characteristic of the second weighting circuit 27.
Denoting the transfer function T(s) as ##EQU2## H(s) is expressed as ##EQU3## in case of a high frequency emphasis characteristic where the DC constitutes the unit gain.
More specifically, when A=1, EQU H(s)=1.
When A=0, ##EQU4## When A=.infin., ##EQU5## Thus, a variable emphasis circuit is constituted, in which the frequency characteristics vary with the gain A of the VCA 24 as shown in FIG. 8.
The gain A can be varied according to the control voltage (Vc), and the opposite transfer function H(s).sup.-1 can be obtained when the gain is 1/A.
FIG. 9 shows an example of such variable emphasis circuit which is formed in an integrated circuit. Reference numeral 31 designates a substrate of the integrated circuit, and numeral 32 a VCA of current input and current output which is formed in the substrate 31. Reference numeral 33 designates an operational amplifier, numeral 34 a variable emphasis input terminal, and numeral 35 an output terminal.